vulama.indd 259 Ivica Vulama: Source rock generative potential and volumetric characteristics of the Kurrachine Dolomite Formation, Hayan Block, central Syrian Palmyrides � AB STRA CT Source rocks from the lower part of the Triassic Kurrachine Dolomite Formation were investigated in order to estab- lish their role in the Petroleum System of the Hayan Block. Recent deep well discoveries accentuate the Triassic Kur- rachine Dolomite Formation as a major play for hydrocarbon accumulations. The characteristic play defi nition is shown for the Hayan Block based on three defi ned elements: petroleum charge system, the source-reservoir forma- tion characteristics and the trap-seal type. According to geochemical analyses, Triassic source rocks are of good gen- erative potential, but they are not abundant (only thin layers were estimated). It was presumed that hydrocarbon gen- eration and migration occurred from deeper Middle-Lower Palaeozoic Formations. Exploration results show that the generative potential of the Triassic source rocks in the Hayan Block was underestimated. Correlation and calibration of the petrophysical parameters from well logs and geochemical analyses show a much larger volume of mature source rocks than that predicted purely from geochemical analyses. The Kurrachine Dolomite Formation resistivity of 20 Ωm is typical of the area, and it represents the threshold value (Net Pay cutoff criteria) between immature and mature source rocks. Net Pay thickness (effective thickness), of mature and generative source rock intervals, exceeds more than 150 m in some wells. A calculated volumetric characteristic of source rocks indicates their high hydrocar- bon generating capability. Intensive tectonic activity formed fractured zones as favourable reservoir rocks, so in some areas, practically no or very short migration pathways exist. Expelled hydrocarbons (within explored area) have not been accumulated into discrete reservoirs, thus the system is generating oil in situ. The results accentuate the existence of a key Triassic Petroleum System in the area of the Hayan Block. This Petro- leum System incorporates the Triassic Kurrachine Dolomite, Amanus Shale Formations and the Permian Amanus Sand Formation. Keywords: Source rock, Triassic, Kurrachine Dolomite Formation, Amanus Shale Formation, Amanus Sand For- mation, Volumetric characteristics, Hydrocarbons generated, Petroleum System, Palmyrides, Syria Source rock generative potential and volumetric characteristics of the Kurrachine Dolomite Formation, Hayan Block, central Syrian Palmyrides � Ivica Vulama 1Exploration Department, INA Oil Company, A. V. Holjevca 10, 10 000 Zagreb, Croatia; (ivica.vulama@ina.hr) doi: 104154/gc.2011.22 Geologia Croatica 64/3 259–272 14 Figs. 2 Tabs. Zagreb 2011 Geologia CroaticaGeologia Croatica 1. INTRODUCTION The characteristics of the Triassic source rocks of the Hayan Block (INA Oil Company exploration Concession) are illus- trated. Recent discoveries at the Hayan Block area indicate that the Kurrachine Dolomite Formation is the most impor- tant source and reservoir rock formation. Geochemical anal- yses estimated only thin layers and a small volume of source rocks, which is not commensurate with the amount of hy- drocarbons discovered. Calculated Net Pay value and volu- metric characteristics of the explored source rocks suggest results different to those known previously. Earlier work on this and the surrounding areas offer some interesting, but to some extent opposing conclusions about the potentiality of Geologia Croatica 64/3Geologia Croatica 260 the source rocks. LEŠKO et al. (2005) studied the source rocks of the Hayan Block in the central Palmyrides. They concluded that the Triassic sediments have good quality but a low quantity of source material, (only thin streaks of source rocks were estimated). ABBOUD et al. (2005) correlated crude oils and source rocks from the Mesopotamian fore- deep, NE Syria and the NE Palmyrides. They defi ned the pre sence of two oil families generated by separate source rock types of different ages and concluded that the major source rocks for oils were the Amanus Shale and/or the Kurra- chine Dolomite Formations. VULAMA & ŠPILJAK VU- LAMA (2007) evaluated and correlated well-log data with geochemical analyses and seismic data. They found that the generative potential of the Kurrachine Dolomite Formation of the Hayan Block was underestimated. According to well log data, mature generative intervals (Net Pay) of the lower part of the Kurrachine Dolomite Formation exceeded 150 m. MICHELS & MALARTRE (2007) defi ned the existence of active source rocks in the Triassic, with two oil types in the Kurrachine Dolomite Formation. They recognized similari- ties in the content of biomarkers and maturity between the Triassic rocks and oils. BARIĆ & SMOLJANOVIĆ (2007) presumed that hydrocarbons within the reservoir rocks of the Hayan block migrated from clastic rocks of Carboniferous (Markada) and Silurian (Tanf) ages. They supposed that Mid- dle Triassic source rocks were of low volume, (thin strata of shale within the reservoir rocks of the Kurrachine Dolomite Formation), with limited generative potential. HIPS & AR- GYELAN (2007) showed that the dark grey dolomite and mudstone of the Kurrachine Dolomite Formation in a nearby area, contained an average Corg of 2 %. The expelled hydro- carbons migrated within the Kurrachine Dolomite, or in some cases, to Permian sandstone sequences. Selective leaching, combined with tectonic activity (fractures), improved the re- servoir potential of the Kurrachine Dolomite Formation. VU LAMA (2009), completed the volumetric calculation of hydrocarbons generated from the lower part of the Kurra- chine Dolomite Formation. Intensive tectonic activity, to- gether with good generative potential, and the maturity of these rocks resulted in fracture zones acting as a favourable, in situ Reservoir/Source Rock system. Similar petrophysical characteristics were determined between the Kurrachine Do- lomite and the Amanus Shale Formation source rocks, and some geochemical correlations of source rock samples and crude oils indicate that the Triassic Petroleum System is a major play in the area of the Hayan Block. The main aim of this work is to show that source rocks of the Triassic Petroleum System have been underestimated in the sense of their generating capability, volume and areal extent. It is recognized that mature source rocks extend over a 220 m interval (VULAMA, 2009). Estimated volumetric values show the high hydrocarbon generating capability of the source rocks. Synthesis of all the evaluated data (well logging, geochemical analyses and 3D seismic), provides a new image and perspective on source rock distribution and hydrocarbon potential of the Kurrachine Dolomite Forma- tion. Finally, the results presented here, suggest that this For- mation generates oil in situ, and represents favourable Pe- troleum System in the area of the Hayan Block. 2. GEOLOGIC SETTING The exploration area of the Hayan Block is situated in the central part of the Syrian Palmyrides, which represents a ty- pi cal intracontinental convergent mountain belt. The Palmy- rides extend from the Anti-Lebanon Mountain and Dead Sea Fault System, in the southwest, towards the Euphrates De- pression in the northeast, a distance of about 400 km over which they sink and disappear. They extend to about 100 km in width reaching heights of approximately 1400 m. To the north, the mountain belt borders the Aleppo Plateau, and the Rutbah Uplift to the south (Fig. 1). Structurally, the Palmyride Trough is composed of the alternation of highs where sediments of Jurassic to Neogene age outcrop, and depressions in which the sediments of Ne- ogene to Recent age occur. Two structural provinces exist. The Southwest province includes the central Palmyride ridge and folded zone of the southeastern Palmyrides between which, the Al-Daww Depression strikes NE–SW (Fig. 1). The Northeast province was separated by the Jihar fault sys- tem, also striking in a NE–SW direction (McBRIDE et al., 1990; CHAIMOV et al., 1990). Exploration wells of the Hayan Block were drilled through deposits of mostly Carboniferous to Miocene age. An inten- sive tectonic history created complex and fractured structures (Fig. 2). The sequence of sediments (source rocks) described (VULAMA, 2009) is most typical for the “B”, “J” and “M” fi elds (Fig. 1). The most frequently drilled sequence starts with Miocene sediments continuing with Oligocene, Eocene, Palaeocene, Upper and Lower Cretaceous sediments. Next are Lower-Middle Jurassic and Upper Triassic sediments, followed by the Middle Triassic (Kurrachine Dolomite For- mation), Lower Triassic (Amanus Shale Formation), Per- mian (Amanus Sand Formation) and fi nally Carboniferous (Markada Formation) sediments. The Kurrachine Dolomite Formation is mainly compos ed of various carbonates. There is a rhythmic alternation of dol- omite, limestone, evaporite and shale. The Syrian General Petroleum Establishment divided this Formation informally into several reservoirs: C2, D1, D2-1 and D2-2. Of these, the C2 reservoir represents a series of rhythmically interca- lated dolomite, limestone, claystone/shale, dolomitic shale and limy dolomite. The D1 reservoir is uniformly developed over the wider area. It comprises microcrystalline limestone, grading to dolomitic limestone and shale, while the D2-1 reservoir consists of limestone, dolomitic limestone, and dol- omite intercalated with shale. A salt interval (D2) separates the D2-1 and D2-2 reservoirs, which have different hydro- dynamic properties. The D2-2 reservoir consists of interca- lations of silty shale and variations of limestone and dolomite sediments. The Lower Triassic (Scythian) – Amanus Shale Formation is composed of argillaceous shale, claystone, sand- stone, siltstone, limestone and dolomite. Sandstones/siltstones with intercalations of shale/claystone and dolomite, predom- inate in the Permian – Amanus Sand Formation. The Car- boniferous Markada Formation is a clastic sequence consist- ing mostly of shale interbedded with claystone. There are also sandstone, limestone, and dolomite beds which are up to se veral meters in thickness (Fig. 3). Geologia Croatica 261 Ivica Vulama: Source rock generative potential and volumetric characteristics of the Kurrachine Dolomite Formation, Hayan Block, central Syrian Palmyrides 2.1. Source rocks Source rocks of the Palmyride Trough were classifi ed as proven and potential (METWALLI et al., 1974; WEBB & THOMPSON, 1994). Proven source rocks occur at three lev- els in Triassic strata and at two levels in rocks of Cretaceous age. The Amanus Shale represents proven source rocks of Triassic age. It is composed of black, calcareous marine mud- stone and shale with a Corg content of up to 20 % (average 8–9 %). The younger, Kurrachine Dolomite Formation (Figs. 2 & 3) is composed of interbedded dark grey dolomite, mud- stone and limestone with a Corg content averaging 2 %. The basal part of the Butmah Formation contains beds of anhy- dritic mudstone, several meters in thickness, with a Corg con- tent of up to 2 %. There are two proven Cretaceous source rocks. In the Euphrates Trough, the Soukhne Formation (Rmah Chert and Arak Marl), is composed of black, bituminous, calcareous, cherty marine mudstones, with a Corg content of up to 8.6 % and the Shiranish Formation with up to 14.3 % Corg (METWALLI et al., 1974; WEBB & THOMPSON, 1994; HIPS & ARGYELAN, 2007). The Carboniferous source rocks of the Markada Forma- tion were considered as having the most potential in the Palmyride Trough and they were proven in the Euphrates Trough. Gas prone shales which passed through phases of the oil window and wet gas/condensate window in the Early Cretaceous occur in the Markada Formation. The source rocks of the Markada Formation (in the Hayan Block) con- tain shale and clayey shale of terrestrial/sapropelic origin (BARIĆ & SMOLJANOVIĆ, 2007). The Early Silurian Tanf Formation is composed of dark partly silicifi ed marine mudstone. It can be correlated with known source rocks of similar age in Oman, Saudi Arabia and South Iran. By analogy, BARIĆ & SMOLJANOVIĆ (2007) presumed that the Tanf Formation could also be the potential source rock in the Hayan Block area. The Tanf Formation is assumed to be overmaturing for oil throughout Syria, except for the shallow parts beneath the Aleppo platform (WEBB & THOMPSON, 1994; MICHELS & MALARTRE, 2007). Hydrocarbon occurrences in Syria were proven in two Petroleum Systems – the Triassic and Cretaceous. The Tri- assic Oil System comprises source rocks of the Amanus Shale, Kurrachine Dolomite, Mulussa and Sergelu Forma- tions. Triassic and Cretaceous carbonates form the reservoirs. These systems actively generated oil in the Late Cretaceous, and wet gas and condensate during the Neogene (WEBB & THOMPSON, 1994). The authors together with MICHELS & MALARTRE (2007) considered the Silurian Oil System of the Tanf Formation and the Carboniferous Markada For- mation speculative, unproved systems. ALSHARHAN & KENDALL (1986) pointed out that the small Butmah oil fi eld in northern Iraq, produces light oil from Kurrachine limestone. This Formation has both reservoir and source rock characteristics and it is composed of limestone with interca- lations of thick-bedded dolomite and silty shale. Source rocks of Carboniferous, Triassic, Cretaceous and Palaeogene age were determined on the Hayan Block (BA- RIĆ & SMOLJANOVIĆ, 2007). Palaeogene marl and lime- stone of the Kermav Formation were immature. Cretaceous Figure 1: Index map of Syria with the main tectonic units, Dead Sea fault sys tem (modifi ed after BREW et al., 2001) and position of the Hayan Block in the Palmyrides (with tecton- ic units and position of oil/gas fi elds: “A”, “B”, “J”, “M” & “P”; modifi ed after VULAMA, 2009). Geologia Croatica 64/3Geologia Croatica 262 deposits of the Shiranish and Soukhne Formations were rich in organic matter, but mostly immature. Occasionally they reached a maturity level of the early catagenetic phase. Mid- dle to late Triassic source rocks of the Kurrachine Dolomite Formation are rich in organic matter. They represent signif- icant source rock in a regional sense (ALSHARHAN & KEN DALL, 1986). Carboniferous shales and clayey shales of the Markada Formation also indicate increased levels of organic matter of terrestrial/sapropelic origin. Dark shales were classifi ed as medium to good gas prone and condensate source rocks (BARIĆ & SMOLJANOVIĆ, 2007). 3. METHODS AND SAMPLES Hydrocarbon source rock parameters (geochemical analy- ses), including the type and amount of kerogene, and thermal maturity data, were assessed by the INA-Naftaplin labora- tory. Net Pay of source rock, volumetric characteristics, ma- turity and other source rocks characteristics have been de- termined through the correlation of well logs, 3D seismic and geochemical analyses. Petrophysical (well log) characteristics of reservoir and source rock parameters have been interpreted on the basis of a broad-spectrum of well logs, which include: GR (natural gamma ray log), NGS (natural gamma ray spectrometry log), AC (acoustic-sonic transit time log), DEN (density log), CN (compensated neutron log), Rt (true formation resistivity) & FMI (Formation MicroScanner Imager) logs. These well log data were used for various cross plots in statistical analysis and for correlation with geochemical and 3D seismic data. Through statistical analyses Discriminant function (D=0) was evaluated to divide source from non-source rocks. 3.1. Geochemical characteristics of source rocks Geochemical analyses of the Triassic succession of “B”, “A” and “P” oil/gas fi elds of the Hayan Block show good gen- erative potential and they are in a thermal oil generating phase. Kerogene is of Type II and III (Tab. 1). 3.2. Petrophysical characteristics of Source and Reservoir Rocks Petrophysical characteristics of source/reservoir rocks from the Kurrachine Dolomite Formation (Hayan Block; D2-2 reservoir, respectively) were interpreted on the basis of nu- merous well-log derived parameters which include: GR, NGS, Rt, AC, DEN, CN and FMI. (Figs. 4–10). Various crossplots (Figs. 6–9) of AC/CN/DEN/Rt/(NGS) logs were made to eva- luate characteristic petrophysical parameters (Figs. 4–6). Evaluation of FMI logs indicates numerous conductive (open) fractures in the zone of source rocks. FMI logs of the Kurrachine Dolomite Formation (also including the Amanus Shale Formation) on “B” and “J” wells, shows typical frac- Table 1: Geochemical parameters of Kurrachine Dolomite Formation source rocks from “B”, “A” and “P2 oil/gas fi elds (see location on Fig. 1). B A P Triassic Corg (%)= 0.48–5.32 Corg = 0.46–1.80 Corg = 0.32–4.72 S2 (mgHC/gCorg)= 0.63–12.11 S2 = 0.49–2.96 S2 = 1.09–30.86 TAI = 3–3+ TAI = 3 TAI = 2–3 (Corg – total organic carbon content; S2 = pyrolisis content of hydrocar- bons in kerogen; TAI= thermal alternation index) Figure 2: Block diagram of the survey area showing the Kurrachine Dolomite Formation and explored source rocks of “B” oil/gas fi eld area (see position of “B” on Fig. 1). Geologia Croatica 263 Ivica Vulama: Source rock generative potential and volumetric characteristics of the Kurrachine Dolomite Formation, Hayan Block, central Syrian Palmyrides ture development in the carbonate zone and associated or- ganic-rich layers – source rocks (high GR). Additionally, the GR (NGS) of observed D2-2 reservoir shows a high uranium (238U) content ranging from 2–13 ppm (Fig. 4). It is assumed that plankton and various organisms absorb uranium salts (ions) that are present in salt water, together with other rare elements. In this way the uranium is concentrated in the or- ganic matter – source rocks (SERRA, 1979; SCHMOCKER, 1981). 4. RESULTS Well log interpretation identifi ed the petrophysical proper- ties of the reservoir and source rocks of the Kurrachine Do- lomite Formation. Correlation and calibration of these petro- physical parameters and geochemical analyses was focused on the lower part of the Kurrachine Dolomite Formation, which shows characteristics typical of mature source rock (Figs. 2–4). Maturity data were provided by geochemical analyses of the Kurrachine Dolomite Formation from the Hayan Block area. As mature source rocks throughout the study area they have low to good generative capabilities, but according to geochemical interpretation they are low in quantity. The characteristic geological development of the Hayan Block area is expressed by thick salt layers (Fig. 3) and sig- nifi cantly fractured reservoirs (Fig. 5). The consequence of these geological conditions was drilling with light oil based mud, blended with numerous organic additives. Geochemi- cal analyses were affected by contamination of cuttings and core samples. The sampling rate of the cuttings taken for geochemical analyses was usually every 20 m, which meant that some samples were not representative and non continu- ous for the purpose of characterization of source rock qual- ity and quantity. Figure 10 shows that samples taken for geo- chemical analyses have high variations in Corg quantity rang ing from 0.56 to 5.32 %, with an average of 1.81 %. Samples taken from low GR readings (assuming low GR = low or- ganic C = poor source rock) shows low Corg values and vice versa. Average Corg values calculated from well logs show a signifi cantly higher amount (2.22 %). Corg was also measured from core samples taken from most favourable (limy, frac- tured) part of the reservoir, giving low readings of Corg (0.08 to 0.30 %). This is also a non-representative, sporadic sam- ple, for the purpose of evaluation of the source rocks. The core was cut from the most favourable zone (lowest GR read- ing, Figs. 4 & 10) for reservoir parameter characterization (VULAMA, 2009). In addition, typical geophysical param- eters were appraised by correlation of well-log (continuous) parameters with geochemical analyses and assorted samples of source rocks. 4.1. Source rock data derived from crossplots The most common combinations used in this research were true resistivity (Rt; Ωm) vs. bulk density (DEN, ρb; g/cm3), and true resistivity vs. sonic transit time (AC, Δt; μs/ft) crossplots (Fig. 6). These combinations of two-component diagrams Figure 3: Schematic lithostratigraphic column of the Hayan Block with lo- cal lithology (after VULAMA, 2009) Geologia Croatica 64/3Geologia Croatica 264 proved to be useful for qualitative rock evaluation, espe- cially when statistically estimated (Figs. 6–9). When, cross- plots were correlated with geochemical parameters, it was possible to determine the Net Pay of the mature source rocks by defi ning their typical resistivity values. The most often used crossplots include the following logs: true (formation) resistivity (Rt) combined with volatile (free) hydrocarbons (S1), vitrinite refl ectance (Ro) and Tmax from "Rock Eval" analysis. Crossplots were used to set the boundary value of formation resistivity at 20 Ωm, thus separating mature from immature source rocks, and setting up the value of volatile hydrocarbons (S1) to be 0.20 mgHC/g of rock, Ro = 0.7 % and Tmax = 436 °C (Figs. 7, 8 & 9). 4.2. Source rock data derived from statistical analysis and crossplots Statistical analysis was applied in order to emphasize the simple classifi cation rules to distinguish source rocks from non-source rocks, based on quantitative well log parameters. These data which determined source rocks were fi ltered through several combinations of crossplots and then addi- tionally checked through statistical analysis. The source rock well log parameters were recorded through organic rich de- posits of the Kurrachine Dolomite Formation. Non-source rock parameters were logged at intervals above and below the source rocks (Figs. 4 & 10). These parameters were di- vided into two classes, based on the geochemical analyses of rock samples: Class 1 = source rock; Class 2 = non-source rock. Discriminant analysis (pseudoregression scheme, D=0) was performed in the statistical analysis (Fig. 6). Geophysi- cal parameters (true resistivity, bulk density, sonic transit time, neutron porosity and natural radioactivity spectrum – uranium, respectively) were used as coordinates to locate the classifi ed rocks. Figure 4: AC/CN/DEN/Rt/ 238U logs of the Kurrachine Dolomite Formation (“M-1” well). Note 238U curve start- ing from 2 ppm and scale of neutron (NPHI) curve start- ing from 0.02 (2 %). Both are min. cut-off values for Source and Reservoir Rock proper- ties. Figure 5: FMI logging of the Kurrachine Dolomite Formation (“B-6” well), showing high fracture porosity of source rocks and adjacent zone. Open- conductive fractures are shown with sinusoids. Geologia Croatica 265 Ivica Vulama: Source rock generative potential and volumetric characteristics of the Kurrachine Dolomite Formation, Hayan Block, central Syrian Palmyrides Figure 6: Upper part of the fi gure shows a crossplot of resistivity (Rt) and sonic in- terval transit time (AC, Δt). D=0 (discri- minant analysis). Points above this line (D=positive) = source rock; points below line (D=negative) = non-source rock. Lower part shows a crossplot of resistiv- ity (Rt) and bulk density (DEN, ρb). Po ints above D=0 line (D=negative) = non- source rock; points below line (D = po - si tive) = source rock. Figure 7: Crossplot of resistivity (Rt) and Tmax showing 20 Ωm and 436 °C as a min. values of mature oil-prone source rocks. Geologia Croatica 64/3Geologia Croatica 266 Finally, according to the petrophysical interpretation, the Kurrachine Dolomite Formation of the Hayan Block area is classifi ed as a signifi cant resource of source and reservoir rocks. Well log analysis of reservoir properties (FMI; Fig. 5) also proved that mature source rocks of the Kurrachine Dol- omite and Amanus Shale Formations are fractured, and they represent in situ source and reservoir rocks with no or short distance migration pathways. Geochemical maturity param- eters of source rocks were calibrated with electrical resistiv- ity logs. A resistivity of 20 Ωm represents the minimum cut- off value separating source rocks from none-source rocks Figure 8: Crossplot of resistivity (Rt) and free (Volatile) hydrocarbons (S1), show- ing 20 Ωm and 0,2 mg HC/g as a min. values of mature oil-prone source rocks. Figure 9: Crossplot of resistivity (Rt) and R0 showing 20 Ωm and vitrinite refl ect- ance (R0= 0.7 %) as a min. values of ma- ture oil-prone source rocks. Table 2: Geophysical and geochemical parameters of the Kurrachine Dol- omite Formation of the Hayan Block evaluated from the correlation of well logs and geochemical analyses. GEOPHYSICAL PARAMETERS GEOCHEMICAL PARAMETERS GR = 48–210 [API] Corg = 0.48–5.32 [%] 238U = 2–13 [ppm] Tmax = 436–448 (455) [oC] AC = 72–90 [μs/ft] S1 = 0.2–2.5 (5.18) [mg HC/g] DEN = 2.38–2.74 [g/cm3] S2 = 0.63–12.11(30,86) [mg HC/g] CN = 2–32 [%] Ro = 0.7–1.25 [%] Rt = 20–>350 [Ωm] TAI = 3–3+ Geologia Croatica 267 Ivica Vulama: Source rock generative potential and volumetric characteristics of the Kurrachine Dolomite Formation, Hayan Block, central Syrian Palmyrides (Figs. 4, 7–9). A uranium content of 2 ppm (238U) was also statistically determined, together with 2 % neutron porosity (CN, NPHI) as the lower (minimum) values for potentially good and mature source rocks and favourable producible reservoir parameters. These parameters were use for calcu- lation of the Net Pay of mature source rocks (Tab. 2). 4.3. Volume of source rocks and generated hydrocarbons Calculation of the volumetric characteristics of source rocks and hydrocarbons generated represents a step forward in the understanding and defi nition of the Petroleum System. Esti- mation of the amount of hydrocarbons generated from active source rocks, represents a basic element for evaluation of the expulsion, migration and accumulation processes affecting the effi ciency of the Petroleum System. Differences between the amount of hydrocarbons generated, compared to the amount in place, can be enormous. It is assumed that only a few percent of oil generated is recoverable oil for most ba- sins, and up to 10 % of recoverable oil is generated in sev- eral sedimentary basins. This part of the article will deal with calculation of the mass of hydrocarbons generated in one part of Triassic Kurrachine Dolomite Formation, of the “B” wells fi eld (Fig. 2, 11, 12 & 13). The SCHMOKER (1994) method is the basis of calculations, which is, in this case, statistically controlled and improved in some parts by using 3D seismic data. The method is based on following elements (steps): 1. Source rock identifi cation (WL, GA), defi nition of mapable boundaries and calculation of the volume of the ho- mogeneous Unit (WL, WL+3D). 2. Calculation of the organic carbon mass in the source rock (WL, GA). 3. Estimation of the mass of hydrocarbons generated per unit mass of organic carbon (WL, GA). 4. Determination of the total mass of hydrocarbons through calculation. (WL=Well Log, GA= Geochemical Analyses, 3D= 3D Seismic Data) The volume, V (cm3), of the Unit is calculated using 3D seismics calibrated with well logs in an effort to extract the Unit from one particular part of the “B” wells of the Hayan Block (Figs. 11–13). The calibration and correlation of the 3D seismic and well logs was undertaken after the well logs were correlated with geochemical analyses. Therefore, we were able to calculate the Net Pay of the mature source rock with higher accuracy. For volumetric calculation of hydrocarbons generated, a potential source rock Unit was bounded by physical fault boundaries to the south and west, but with a fi ctive ”contact” at –3 000 m to the north (Fig. 11). The thickness of the Unit was set at 70 m. Following this setup parameters, the vol- ume of the Unit was calculated as (V) = 1.4x109 m3 (1.4x1015 cm3; Fig. 12). The second step (Fig. 12) is to calculate the mass of or- ganic carbon M(gCorg), formula (1) M(gCorg) = [Corg(wt%)/100] x [ρb(g/cm3) x V(cm3)] (1) The next step is to determine the mass of hydrocarbons generated per unit mass of organic carbon, R(mgHC/gCorg) formula (2). The data needed to calculate R are the original hydrogen index HI0 (mgHC/gCorg), prior to any hydrocarbon generation, and the present hydrogen index HIp (mgHC/ gCorg) of the source rock. The difference between these two indexes indices approximates the mass of hydrocarbons gen- erated per gram Corg. R(mgHC/gCorg) = HI0 (mgHC/gCorg) – HIp (mgHC/gCorg) (2) Figure 10: Sampled intervals (rectangles) for geochemical analyses from the B-5 well show high discrepancies in Corg concentration. Samples from the high GR zone show high concentration of organic matter and vice ver- sa. The average value of 1.81 % is not a representative value for the lower part of the Triassic Kurrachine Dolomite Formation. Calculated average Corg concentration from well logs is 2.22 %. Geologia Croatica 64/3Geologia Croatica 268 The equation equates the decline in generation potential to hydrocarbons actually generated. The explored area of the Hayan Block has an initial index of 650 mgHC/gCorg derived from immature source rocks of the Kurrachine Dolomite Formation from “P” wells. The fi nal step is to compute the total mass of hydrocar- bons generated HCG (kgHC) computing the data from fi rst two steps: HCG (kgHC) = R(mgHC/gCorg) x M(gCorg) x 10–6 ( kg/mg) (3) Following these equations, the volumetric calculation method was applied to one part (Unit) of the Kurrachine Dolomite Formation (Figs. 10, 11 & 12). These parameters were calculated: – V = 1.4 x 1015 cm3 (1.4x109 m3), volume from 3D seis- mic) – Corg = 1.84 mgHC/gCorg (average data from GA – geo- chemical analysis), – ρb = 2.45g/cm3 (WL) According to equation (1) mass equals: – M = 6.32x1013 Corg (1) For the second step present Hip is 450 mgHC/gCorg (GA), and initial Hi0 is 650 mgHC/gCorg, according to equation (2): – R = 200 mgHC/gCorg (2) Concluding with equation (3): – HCG = 1.27x 1010 kgHC (3) Converted to oil equivalent, using SCHMOCKER’s, (1994) graph, produces: 0.8 x 108 bbl or 12.7 x 106 m3 or 10.7 x 106 tons of oil equivalent. Finally, taking into consideration evaluation of Net Pay of mature source rocks with discriminant factors Rt= 20 Ωm, 238U= 2 ppm and CN= 2 %, (VULAMA & ŠPILJAK VU- LAMA, 2007), the result must be reduced by approximately 20 % to HCG = 10.16 x 106 m3 or 8.6 x 106 (36 API) tons of oil equivalent. Figure 12: Seismic section through the “B” oil/gas fi eld with extracted source rock Unit (yellow) and 3D shape (magenta) of the source rock body, 70 m thick (see location on Fig. 1). Figure 11: Structure map on the top of the mature source rocks of the Kur- rachine Dolomite Forma- tion of “B” oil/gas fi eld (see location on Fig. 1). Geologia Croatica 269 Ivica Vulama: Source rock generative potential and volumetric characteristics of the Kurrachine Dolomite Formation, Hayan Block, central Syrian Palmyrides According to McDOWELL (1975), recoverable oil is in the range of 10 % of the total oil generated, for only a few sedimentary basins, and is only a few percent for most ba- sins. The difference between the amount of hydrocarbons generated, compared to the amount of oil in place can be enormous, due to the ineffi ciencies associated with the ex- pulsion, migration, and trapping of hydrocarbons are often large. As SCHMOCKER (1994) pointed out, the parameters necessary to calculate the amount of hydrocarbons generated are poorly constrained, resulting in the calculation error. Al- though disadvantageous, existing errors do not necessarily negate the value of volumetric hydrocarbon calculations be- cause it is useful to know the range of the oil equivalent e.g. 1–10–100 millions tons of equivalent oil. 5. DISCUSSION Recent hydrocarbon discoveries (“A”, “B”, “J”, “M” and “P“ Oil/Gas fi elds) in the Hayan Block showed that the Tri- assic Petroleum System is a major play in that area. The hy- drocarbon potential of the Kurrachine Dolomite Formation, (both reservoir and source rock properties as a system in situ) in previous explorations was not considered as signifi - cant. Herein, results and conclusions from previous work by authors from the INA Oil Company or their consultants are discussed. BARIĆ & SMOLJANOVIĆ (2007) concluded that hy- drocarbons of the Kurrachine Dolomite Formation were probably of secondary origin (Carboniferous and Silurian ages). This secondary, migrated origin of hydrocarbons is not supported by the geochemical (“Rock-Eval”) analyzed data. These authors only presumed that the main regional source rocks were marine shale pellet deposits of the Markada and Tanf Formations. Data from geochemical analyses (with high HI content (450–620 mgHC/g; in P-wells area even 555–650 for imma- ture source rock in that area), and other geochemical param- eters (Tab. I) support a high generative potential for the Kur- rachine Dolomite Formation. Following, in their conclusion, authors (BARIĆ & SMOLJANOVIĆ, 2007) pointed out that Middle Triassic source rocks of the Kurrachine Dolomite Formation were thin streaks of shale within the reservoir rocks of that For- mation, with limited generative possibility. This conclusion is also not acceptable as VULAMA & ŠPILJAK VULAMA (2007) confi rmed that more than 150 m of Net Pay of mature source rocks exists only in the lower part of the reservoir. These source rocks layers are not thin at all (Figs. 4 & 10). BARIĆ & SMOLJANOVIĆ (2007) emphasized that these source rock sequences (thin layers) are thermally mature, and that they reached the main oil window phase. The older sediments of the Amanus Shale and Amanus Sand Forma- tion, have restricted organic matter content (Corg < 0.5 %). The conclusion of the existence of thin layers of source rocks was also addressed by previous work of VULAMA & ŠPILJAK VULAMA (2007) where it was shown that a lot of the geochemical tested samples were from non represent- ative parts of reservoirs. This resulted in a much lower Corg content and misleading conclusion about the low Corg con- tent, low capacity of source rocks and the conclusion of the existence of thin layers of source rocks throughout the entire Kurrachine Dolomite Formation. The Markada Formation, in the area of the “B” wells, was only sampled by one deep well and both reservoir and source rock capacity, was low to medium and it has not been suffi ciently explored. MICHELIS & MALALTRE (2007) confi rmed the exist- ence of an active Triassic source rock (with some reserva- tions). Strong similarities in geochemical characteristics be- tween the oils from the Kurrachine Dolomite reservoirs and rock extracts from the Triassic were determined. They pro- posed that oils from the Kurrachine Dolomite reservoirs were sourced from Triassic source rocks. MICHELIS & MALA- LTRE (2007) pointed out that they identify only oil prone source rock from the “Rock-Eval” data of the Kurrachine Dolomite Formation from the “P” well. Markada “Rock- Eval” data indicate rather gas-prone immature to highly ma- ture source rocks (Ro= 1.31–1.54 %, TAI = 3–3+, Corg = 0.82– 1.07 %). Concerning the Tanf Formation, they hypothesize that as it is deeply buried in the area, it must be over mature, (in addition – only one well drilled the Tanf Formation, some 45 km SE from the “B” wells in the Southern Palmyrides). Finally, in their conclusion they pointed out that they didn’t have enough good quality samples to analyze. The samples were either very altered by meteoric water input, i.e. water washing and biodegradation or contaminated by drilling fl uid. This conclusion can also be related to the evaluation of results from VULAMA & ŠPILJAK VULAMA (2007), who indicated the problems of contaminated samples which were mostly taken from non-representative parts of the Kurrachine Dolomite Formation (non-source rocks zones) for geochem- ical analyses. This problem was solved after drilling the “M” well (Fig. 4), where geochemical analyses were focused to se- lected zones with good source rock characteristics inter- Figure 13: Interpreted detail of seismic profi le (see C-D, on Fig. 11) of Kur- rachine Dolomite Formation (B-6 well). Geologia Croatica 64/3Geologia Croatica 270 preted from well logs (VULAMA, 2009). The samples were selected according to several criteria selected from correla- tion of well logs and geochemical analyses, in order to rep- resent the most signifi cant parts of the reservoir with a spe- cial focus on formations bearing source rock potentiality. This means that the chosen rock samples have “Rock-Eval” characteristics of good to excellent source rocks, over an in- terval of 220 m, but only in the lower part of reservoir. Se- lected representative samples helped to avoid the rock sam- ples of equivalent facies but without proven oil potential (rock samples with unfavorable “Rock-Eval” characteris- tics). Finally it can be concluded that geochemical analyses of the Kurrachine Dolomite Formation from wells of the Hayan Block encountered contamination problems because of drilling with oil based mud and added organic additives. Non-representative samples collected according to a program of ~20 m interval sampling were mostly out with the best concentrations of organic matter (source rocks, especially cores; Fig. 4 & 10). This resulted in the conclusion of the existence of only thin layers of source rocks with low hydro- carbon generating capability. By interpreting continuous data from well logs, it was established that the entire Kurrachine Dolomite Formation, especially the lower part, represents mature source rock with an effective Net Pay thickness of more than 150 m. This research defi ned marginal geophysical (well log- ging) parameters for mature source/reservoir rocks, the most important being the value of formation resistivity of 20 % Ωm, and 238U content of 2 ppm and CN porosity of 2 %. Favourable tectonic and sedimentation conditions (car- bonates and shales sealed with thick salt-evaporite layers), signifi cantly affected and shaped this area. Intensive tectonic activity played a double function – it shaped structures and traps, and formed fractured zones – reservoirs for hydrocar- bon accumulation (Fig. 13). Mature and fractured source rocks, represent a signifi cant hydrocarbon generative and reservoir system in situ (oil in place), with no or minor mi- gration path ways, as was regionally recognized in some sur- rounding areas. Reservoir pressure measurements in the “B” fi eld well contributed to these results (Fig. 14). The fi gure shows that three different, separated hydrodynamic units have various and mixed hydrocarbon saturation. If sources for these hy- drocarbons were from Silurian and Carboniferous Forma- tions, deeply buried in this area (1000–1500 m deeper), it would be diffi cult to explain how the hydrocarbons migrated through these regional barriers in such amounts with such a high pressure difference and mixed composition. Bearing in mind possible migrations through faults, we must take into consideration that deeper rocks are capable to generate only dry gas – as in the Markada Formation, or they can be over- mature – as in the Tanf Formation. In the area of the Hayan Block, several major – regional barriers prevent migrations from deeper sediments, and they generate separated hydro- dynamic units. The fi rst barrier is the regional shale layer of the Amanus Sand Formation, which is up to 300 m thick. Then there are two very thick salt/evaporite layers which may also exceed several hundred meters in thickness (Figs. 13 & 14). Figure 14: Pressure gradients of Kurrachine Dolomite, Amanus Shale and Amanus Sand Formations (“B” oil/gas fi eld). Geologia Croatica 271 Ivica Vulama: Source rock generative potential and volumetric characteristics of the Kurrachine Dolomite Formation, Hayan Block, central Syrian Palmyrides Results presented here lead to the conclusion that an ac- tive Triassic Petroleum System exists in the area of the Hayan Block. Hydrocarbon accumulations in Syria were already proven in the Triassic Oil System which includes source rocks of the Amanus Shale, Kurrachine Dolomite, Mulussa and Sergelu Formations and Triassic and Cretaceous carbon- ate reservoirs (METWALLI et al., 1974, WEBB & THOMP- SON, 1994; ABBOUD et al., 2005). In order to prove the Triassic Petroleum System as an important source and res- ervoir rock complex over the whole area, some additional work is necessary (i.e. the upper part of Kurrachine Dolo- mite Formation also shows strong mature source rock char- acteristics). 6. CONCLUSION Investigation of the Triassic Kurrachine Dolomite Formation elements (source, reservoir, seal rock type and trap confi gu- ration), was essential for Petroleum System and play defi ni- tion. It was found that according to geochemical analyses, the source rock potential was underestimated. Geochemical ana- lyses predicted only thin layers of source rocks which could not generate suffi cient amounts of hydrocarbons present in the Triassic/Permian reservoirs. Moreover, it was presumed that hydrocarbons in Triassic reservoirs were generated from deeper Palaeozoic formations (BARIĆ & SMOLJANOVIĆ, 2007). However, Net Pay of mature source rocks calculated from well logs showed much higher thicknesses of mature source rocks. A Formation resistivity of 20 Ωm is typical of the area and it represents the threshold value for mature source rocks and Net Pay calculating criteria. Source rocks were also marked on 3D seismic sections. The calculated volume (from 3D seismic) and amount of generated hydrocarbons on the one part of the “B” wells fi eld highly supports the fi nal results and conclusions. Triassic source rocks of the Kurrachine Dolomite For- mation in the area of the Hayan Block have signifi cant Net Pay of mature source rock, exceeding more than 150 m (through 220 m gross interval of source rock, which belongs only to the lower part of the reservoir). It was concluded that mature fractured source rock represents an in situ source and reservoir rock, and that either no hydrocarbon migration pathways exist or they were over very short distances. Evaluated and compared well log petrophysical similari- ties between the Kurrachine Dolomite and Amanus Shale For- mations source rocks, supported with strong geochemical matching of source rock/oil samples, set the Triassic Petroleum System as a mayor play in the area of the Hayan Block. The Triassic Petroleum System is represented by the fractured source rock and reservoir system in situ – Triassic Kurrachine Dolomite and Amanus Shale Formations, respec- tively, and in some parts conventional Permian Amanus Sand reservoirs. Thick salt-evaporite layers represent the seals: D2 salt and salt on top of the Kurrachine Dolomite Forma- tion – bottom part of the Kurrachine Anhydrite Formation. ACKNOWLEDGEMENT This paper represents one part of my PhD thesis which was supported by INA Oil Company, with permission and thanks to Josip KRIŽ, MSc. Željko IVKOVIĆ and MSc. Niko DALIĆ. I am thankful to the review- ers Prof. Dr. Josipa VELIĆ and Dr. György POGÁCSÁS for their help- ful guidance, supportive comments and suggestions that helped to im- prove this manuscript. Special thanks go to Vesna ŠPILJAK VULAMA for her technical sup- port, maps and fi gures drawings and being co-author in previous re- search. 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